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Synthetic fuel orsynfuel is aliquid fuel, or sometimesgaseous fuel, obtained fromsyngas, a mixture ofcarbon monoxide andhydrogen, in which the syngas was derived from gasification of solid feedstocks such ascoal orbiomass or by reforming ofnatural gas.[1][2]
Common ways for refining synthetic fuels include theFischer–Tropsch conversion,[3][4][better source needed]methanol to gasoline conversion,[5][better source needed] or directcoal liquefaction.[6][better source needed]
There is a range of meanings for the terms 'synthetic fuel' or 'synfuel'.
Synthetic fuels are produced by the chemical process ofconversion.[12] Conversion methods could be direct conversion into liquid transportation fuels, or indirect conversion, in which the source substance is converted initially intosyngas which then goes through additional conversion processes to become liquid fuels.[7] Basic conversion methods includecarbonization andpyrolysis,hydrogenation, andthermal dissolution.[15]
The process of direct conversion of coal to synthetic fuel originally developed in Germany.[16]Friedrich Bergius developed theBergius process, which received a patent in 1913. Karl Goldschmidt invited Bergius to build an industrial plant at his factory, the Th. Goldschmidt AG (part ofEvonik Industries from 2007), in 1914.[17] Production began in 1919.[18][citation needed]
Indirect coal conversion (where coal is gasified and then converted to synthetic fuels) was also developed in Germany - byFranz Fischer andHans Tropsch in 1923.[16] DuringWorld War II (1939-1945), Germany used synthetic-oil manufacturing (German:Kohleverflüssigung) to produce substitute (Ersatz) oil products by using theBergius process (from coal), theFischer–Tropsch process (water gas), and other methods (Zeitz used the TTH and MTH processes).[19][20]In 1931 the BritishDepartment of Scientific and Industrial Research located inGreenwich, England, set up a small facility where hydrogen gas was combined with coal at extremely high pressures to make a synthetic fuel.[21]
The Bergius process plants became[when?]Nazi Germany's primary source of high-grade aviation gasoline, synthetic oil,synthetic rubber, syntheticmethanol, syntheticammonia, andnitric acid. Nearly one third of the Bergius production came from plants inPölitz (Polish:Police) andLeuna, with 1/3 more in five other plants (Ludwigshafen had a much smaller Bergius plant[22] which improved "gasoline quality by dehydrogenation" using the DHD process).[20]
Synthetic fuel grades included "T.L.[jet] fuel", "first quality aviation gasoline", "aviation base gasoline", and "gasoline - middle oil";[20] and "producer gas" and diesel were synthesized for fuel as well (converted armored tanks, for example, used producer gas).[19]: 4, s2 By early 1944 German synthetic-fuel production had reached more than 124,000 barrels per day (19,700 m3/d) from 25 plants,[23] including 10 in theRuhr Area.[24]: 239 In 1937 the four central Germanylignite coal plants atBöhlen, Leuna,Magdeburg/Rothensee, and Zeitz, along with theRuhr Areabituminous coal plant at Scholven/Buer, produced 4.8 million barrels (760×10^3 m3) of fuel. Four new hydrogenation plants (German:Hydrierwerke) were subsequently erected atBottrop-Welheim (which used "Bituminouscoal tar pitch"),[20]Gelsenkirchen (Nordstern), Pölitz, and, at 200,000 tons/yr[20]Wesseling.[25] Nordstern and Pölitz/Stettin used bituminous coal, as did the newBlechhammer plants.[20]Heydebreck synthesized food oil, which was tested onconcentration camp prisoners.[26] AfterAllied bombing of Germany's synthetic-fuel production plants (especially in May to June 1944), theGeilenberg Special Staff used 350,000 mostly foreignforced-laborers to reconstruct the bombed synthetic-oil plants,[24]: 210, 224 and, in an emergency decentralization program, theMineralölsicherungsplan [de] (1944-1945), to build 7 underground hydrogenation plants with bombing protection (none were completed). (Planners had rejected an earlier such proposal, expecting thatAxis forces would win the war before thebunkers would be completed.)[22] In July 1944 the "Cuckoo" project underground synthetic-oil plant (800,000 m2) was being "carved out of theHimmelsburg" north of theMittelwerk, but the plant remained unfinished at the end of World War II.[19] Production of synthetic fuel became even more vital for Nazi Germany when SovietRed Army forces occupied thePloiești oilfields in Romania on 24 August 1944, denying Germany access to its most important natural oil source.
Indirect Fischer–Tropsch ("FT") technologies were brought to theUnited States after World War II, and a 7,000 barrels per day (1,100 m3/d) plant was designed byHRI and built inBrownsville, Texas. The plant represented the first commercial use of high-temperature Fischer–Tropsch conversion. It operated from 1950 to 1955, when it was shut down after the price of oil dropped due to enhanced production and huge discoveries in the Middle East.[16]
In 1949 theU.S. Bureau of Mines built and operated a demonstration plant for converting coal to gasoline inLouisiana, Missouri.[27] Direct coal conversion plants were also developed in the US after World War II, including a 3 TPD plant inLawrenceville, New Jersey, and a 250-600 TPD Plant inCatlettsburg, Kentucky.[28]
In later decades theRepublic of South Africa established astate oil company including a largesynthetic fuel establishment.[citation needed]
The numerous processes that can be used to produce synthetic fuels broadly fall into three categories: Indirect, Direct, and Biofuel processes.[dubious –discuss]
Indirect conversion has the widest deployment worldwide, with global production totaling around 260,000 barrels per day (41,000 m3/d), and many additional projects under active development.[citation needed]
Indirect conversion broadly refers to a process in which biomass, coal, or natural gas is converted to a mix ofhydrogen andcarbon monoxide known as syngas either throughgasification orsteam methane reforming, and that syngas is processed into a liquid transportation fuel using one of a number of different conversion techniques depending on the desired end product.[29]
The primary technologies that produce synthetic fuel from syngas areFischer–Tropsch synthesis and theMobil process (also known as Methanol-To-Gasoline, or MTG). In the Fischer–Tropsch process syngas reacts in the presence of a catalyst, transforming into liquid products (primarilydiesel fuel andjet fuel) and potentially waxes (depending on the FT process employed).[30]
The process of producing synfuels through indirect conversion is often referred to as coal-to-liquids (CTL),gas-to-liquids (GTL) orbiomass-to-liquids (BTL), depending on the initial feedstock. At least three projects (Ohio River Clean Fuels, Illinois Clean Fuels, and Rentech Natchez) are combining coal and biomass feedstocks, creating hybrid-feedstock synthetic fuels known as Coal and Biomass To Liquids (CBTL).[31]
Indirect conversion process technologies can also be used to produce hydrogen, potentially for use in fuel cell vehicles, either as slipstream co-product, or as a primary output.[32]
Direct conversion refers to processes in which coal or biomass feedstocks are converted directly into intermediate or final products, avoiding the conversion to syngas viagasification. Direct conversion processes can be broadly broken up into two different methods: Pyrolysis and carbonization, and hydrogenation.[33]
One of the main methods of direct conversion of coal to liquids by hydrogenation process is the Bergius process.[34] In this process, coal is liquefied by heating in the presence of hydrogen gas (hydrogenation). Dry coal is mixed with heavy oil recycled from the process.Catalysts are typically added to the mixture. The reaction occurs at between 400 °C (752 °F) to 500 °C (932 °F) and 20 to 70 MPa hydrogen pressure.[35] The reaction can be summarized as follows:[35]
AfterWorld War I several plants were built in Germany; these plants were extensively used duringWorld War II to supply Germany with fuel and lubricants.[36]
The Kohleoel Process, developed in Germany byRuhrkohle andVEBA, was used in the demonstration plant with a capacity of 200 tons oflignite per day, built inBottrop, Germany. This plant operated from 1981 to 1987. In this process, coal is mixed with a recycled solvent and an iron catalyst. After preheating and pressurizing, H2 is added. The process takes place in a tubular reactor at a pressure of 300 bar and a temperature of 470 °C (880 °F).[37] This process has also been explored bySASOL in South Africa.
In the 1970-1980s, the Japanese companiesNippon Kokan,Sumitomo Metal Industries andMitsubishi Heavy Industries developed the NEDOL process. In this process, a mixture of coal and a recycled solvent is heated in the presence of an iron-based catalyst and H2. The reaction takes place in a tubular reactor at a temperature between 430 °C (810 °F) and 465 °C (870 °F) at a pressure of 150-200 bar. The produced oil has low quality and requires intensive upgrading.[37] The H-Coal process, developed by Hydrocarbon Research, Inc., in 1963, mixes pulverized coal with recycled liquids, hydrogen and a catalyst in theebullated bed reactor. The advantages of this process are that dissolution and oil upgrading take place in a single reactor, the products have a high H:C ratio and a fast reaction time, while the main disadvantages are high gas yield, high hydrogen consumption and the produced oil is only suitable as boiler oil because of impurities.[38]
The SRC-I and SRC-II (Solvent Refined Coal) processes were developed byGulf Oil and implemented as pilot plants in the United States in the 1960s and 1970s.[37] The Nuclear Utility Services Corporation developed the hydrogenation process which was patented by Wilburn C. Schroeder in 1976. The process involved dried, pulverized coal mixed with roughly 1wt%molybdenum catalysts.[12] Hydrogenation occurred at a high temperature and pressure, with syngas produced in a separate gasifier. The process ultimately yielded a synthetic crude product,Naphtha, a limited amount of C3/C4 gas, light-medium weight liquids (C5-C10) suitable for use as fuels, small amounts of NH3 and significant amounts of CO2.[39] Other single-stage hydrogenation processes are theExxon donor solvent process, the Imhausen High-pressure Process, and the Conoco Zinc Chloride Process.[37]
A number of two-stage direct liquefaction processes have been developed. After the 1980s only the Catalytic Two-stage Liquefaction Process, modified from the H-Coal Process; the Liquid Solvent Extraction Process byBritish Coal; and the Brown Coal Liquefaction Process of Japan have been developed.[37]
Chevron Corporation developed a process invented by Joel W. Rosenthal called the Chevron Coal Liquefaction Process (CCLP). It is unique due to the close-coupling of the non-catalytic dissolver and the catalytic hydroprocessing unit. The oil produced had properties that were unique when compared to other coal oils; it was lighter and had far fewer heteroatom impurities. The process was scaled-up to a 6 ton per day level, but not proven commercially.
There are a number of different carbonization processes. The carbonization conversion occurs throughpyrolysis ordestructive distillation, and it produces condensablecoal tar, oil and water vapor, non-condensablesynthetic gas, and a solid residue-char. The condensed coal tar and oil are then further processed by hydrogenation to removesulfur andnitrogen species, after which they are processed into fuels.[38]
The typical example of carbonization is theKarrick process. The process was invented byLewis Cass Karrick in the 1920s. The Karrick process is a low-temperaturecarbonization process, where coal is heated at 680 °F (360 °C) to 1,380 °F (750 °C) in the absence of air. These temperatures optimize the production of coal tars richer in lighter hydrocarbons than normal coal tar. However, the produced liquids are mostly a by-product and the main product is semi-coke, a solid and smokeless fuel.[40]
The COED Process, developed byFMC Corporation, uses afluidized bed for processing, in combination with increasing temperature, through four stages of pyrolysis. Heat is transferred by hot gases produced by combustion of part of the produced char. A modification of this process, the COGAS Process, involves the addition of gasification of char.[38] The TOSCOAL Process, an analogue to theTOSCO II oil shale retorting process andLurgi-Ruhrgas process, which is also used for theshale oil extraction, uses hot recycled solids for the heat transfer.[38]
Liquid yields of pyrolysis and Karrick processes are generally low for practical use for synthetic liquid fuel production.[40] Furthermore, the resulting liquids are of low quality and require further treatment before they can be used as motor fuels. In summary, there is little possibility that this process will yield economically viable volumes of liquid fuel.[40]
One example of a Biofuel-based synthetic fuel process is Hydrotreated Renewable Jet (HRJ) fuel.There are a number of variants of these processes under development, and the testing and certification process for HRJ aviation fuels is beginning.[41][42]
There are two such process under development byUOP. One using solid biomass feedstocks, and one usingbio-oil and fats. The process using solid second-generation biomass sources such as switchgrass or woodybiomass uses pyrolysis to produce a bio-oil, which is then catalytically stabilized and deoxygenated to produce a jet-range fuel. The process using natural oils and fats goes through a deoxygenation process, followed by hydrocracking and isomerization to produce a renewableSynthetic Paraffinic Kerosene jet fuel.[43]
Synthetic crude may also be created byupgradingbitumen (a tar like substance found inoil sands), or synthesizing liquidhydrocarbons from oil shale. There are a number of processesextracting shale oil (synthetic crude oil) fromoil shale by pyrolysis, hydrogenation, or thermal dissolution.[15][44]
Tetraethyllead was the default additive for increasing octane in gasoline, in particular important to synthetic fuels like in 3rd Reich Germany, having acquired this manufacturing process and equipment from the USA viaDuPont according to Prof. Dr. Anthony C. Sutton. Tetraethyllead is disbanded for terrestrial applications because of thetoxicity of lead.
Worldwide commercial synthetic fuels plant capacity is over 240,000 barrels per day (38,000 m3/d), including indirect conversion Fischer–Tropsch plants in South Africa (Mossgas,Secunda CTL), Qatar (Oryx GTL), and Malaysia (Shell Bintulu), and a Mobil process (Methanol to petrol) plant in New Zealand.[7][45] Synthetic fuel plant capacity is approximately 0.24% of the 100 million barrel per day crude oil refining capacity worldwide.[46]
Sasol, a company based in South Africa operates the world's only commercial Fischer–Tropsch coal-to-liquids facility atSecunda, with a capacity of 150,000 barrels per day (24,000 m3/d).[47] British companyZero, co-founded by former F1 technical directorPaddy Lowe, has developed a solution it terms 'petrosynthesis' to develop synthetic fuels and in 2022 it began work on a demonstration production plant[48] at Bicester Heritage near Oxford.
The economics of synthetic fuel manufacture vary greatly depending the feedstock used, the precise process employed, site characteristics such as feedstock and transportation costs, and the cost of additional equipment required to control emissions. The examples described below indicate a wide range of production costs between $20/BBL for large-scale gas-to-liquids, to as much as $240/BBL for small-scale biomass-to-liquids and carbon capture and sequestration.[31]
In order to be economically viable, projects must do much better than just being competitive head-to-head with oil. They must also generate a sufficient return on investment to justify the capital investment in the project.[31]
A central consideration for the development of synthetic fuel is the security factor of securing domestic fuel supply from domestic biomass and coal. Nations that are rich in biomass and coal can use synthetic fuel to offset their use of petroleum derived fuels and foreign oil.[49]
The environmental footprint of a given synthetic fuel varies greatly depending on which process is employed, what feedstock is used, what pollution controls are employed, and what the transportation distance and method are for both feedstock procurement and end-product distribution.[31]
In many locations, project development will not be possible due to permitting restrictions if a process design is chosen that does not meet local requirements for clean air, water, and increasingly, lifecycle carbon emissions.[50][51]
Among different indirect FT synthetic fuels production technologies, potential emissions ofgreenhouse gases vary greatly.Coal to liquids ("CTL") withoutcarbon capture and sequestration ("CCS") is expected to result in a significantly higher carbon footprint than conventional petroleum-derived fuels (+147%).[31] On the other hand, biomass-to-liquids with CCS could deliver a 358% reduction in lifecyclegreenhouse gas emissions.[31] Both of these plants fundamentally usegasification and FT conversion synthetic fuels technology, but they deliver wildly divergent environmental footprints.[citation needed]
Generally, CTL without CCS has a higher greenhouse gas footprint. CTL with CCS has a 9-15% reduction in lifecycle greenhouse gas emissions compared to that of petroleum derived diesel.[31][52]
CBTL+CCS plants that blend biomass alongside coal whilesequestering carbon do progressively better the more biomass is added. Depending on the type of biomass, the assumptions about root storage, and the transportation logistics, at conservatively 40% biomass alongside coal, CBTL+CCS plants achieve a neutral lifecycle greenhouse gas footprint. At more than 40% biomass, they begin to go lifecycle negative, and effectively store carbon in the ground for every gallon of fuels that they produce.[31]
Ultimately BTL plants employing CCS could store massive amounts of carbon while producing transportation fuels from sustainably produced biomass feedstocks, although there are a number of significant economic hurdles, and a few technical hurdles that would have to be overcome to enable the development of such facilities.[31]
Serious consideration must also be given to the type and method of feedstock procurement for either the coal or biomass used in such facilities, as reckless development could exacerbate environmental problems caused bymountaintop removal mining, land use change, fertilizer runoff,food vs. fuels concerns, or many other potential factors. Or they could not, depending entirely on project-specific factors on a plant-by-plant basis.[citation needed]
A study from U.S. Department of Energy National Energy Technology Laboratory with much more in-depth information of CBTL life-cycle emissions "Affordable Low Carbon Diesel from Domestic Coal and Biomass".[31]
Hybrid hydrogen-carbon processes have also been proposed recently[53] as another closed-carbon cycle alternative, combining'clean' electricity, recycled CO, H2 and captured CO2 with biomass as inputs as a way of reducing the biomass needed.[citation needed]
The fuels produced by the various synthetic fuels process also have a wide range of potential environmental performance, though they tend to be very uniform based on the type of synthetic fuels process used (i.e. the tailpipe emissions characteristics of Fischer–Tropsch diesel tend to be the same, though their lifecycle greenhouse gas footprint can vary substantially based on which plant produced the fuel, depending on feedstock and plant level sequestration considerations.)[citation needed]
In particular, Fischer–Tropsch diesel and jet fuels deliver dramatic across-the-board reductions in all major criteria pollutants such as SOx, NOx, Particulate Matter, and Hydrocarbon emissions.[54] These fuels, because of their high level of purity and lack of contaminants, allow the use of advanced emissions control equipment. In a 2005 dynamometer study simulating urban driving the combination was shown to virtually eliminate HC, CO, and PM emissions from diesel trucks with a 10% increase in fuel consumption using a Shell gas to liquid fuel fitted with a combination particulate filter and catalytic converter compared to the same trucks unmodified using California Air Resource Board diesel fuel .[55]
In testimony before the Subcommittee on Energy and Environment of the U.S. House of Representatives the following statement was made by a senior scientist from Rentech:
F-T fuels offer numerous benefits to aviation users. The first is an immediate reduction in particulate emissions. F-T jet fuel has been shown in laboratory combusters and engines to reduce PM emissions by 96% at idle and 78% under cruise operation. Validation of the reduction in other turbine engine emissions is still under way. Concurrent to the PM reductions is an immediate reduction in CO2 emissions from F-T fuel. F-T fuels inherently reduce CO2 emissions because they have higher energy content per carbon content of the fuel, and the fuel is less dense than conventional jet fuel allowing aircraft to fly further on the same load of fuel.[56]
The "cleanness" of these FT synthetic fuels is further demonstrated by the fact that they are sufficiently non-toxic and environmentally benign as to be considered biodegradable. This owes primarily to the near-absence of sulfur and extremely low level of aromatics present in the fuel.[57]
In 2023, a study published by the NATO Energy Security Centre of Excellence, concluded that synthetic FT fuels offer one of the most promising decarbonization pathways for military mobility across the land, sea and air domains.[58]
One concern commonly raised about the development of synthetic fuels plants is sustainability. Fundamentally, transitioning from oil to coal or natural gas for transportation fuels production is a transition from one inherently depletable geologically limited resource to another.
One of the positive defining characteristics of synthetic fuels production is the ability to use multiple feedstocks (coal, gas, or biomass) to produce the same product from the same plant. In the case of hybrid BCTL plants, some facilities are already planning to use a significant biomass component alongside coal. Ultimately, given the right location with good biomass availability, and sufficiently high oil prices, synthetic fuels plants can be transitioned from coal or gas, over to a 100% biomass feedstock. This provides a path forward towards a renewable fuel source and possibly more sustainable, even if the plant originally produced fuels solely from coal, making the infrastructure forwards-compatible even if the original fossil feedstock runs out.[citation needed]
Some synthetic fuels processes can be converted to sustainable production practices more easily than others, depending on the process equipment selected. This is an important design consideration as these facilities are planned and implemented, as additional room must be left in the plant layout to accommodate whatever future plant change requirements in terms of materials handling and gasification might be necessary to accommodate a future change in production profile.[citation needed]
Electrofuels, also known ase-fuels orsynthetic fuels, are a type of drop-in replacement fuel. They are manufactured using captured carbon dioxide or carbon monoxide, together withhydrogen obtained from sustainable electricity sources such as wind, solar and nuclear power.[59]
The process uses carbon dioxide in manufacturing and releases around the same amount of carbon dioxide into the air when the fuel is burned, for an overall low carbon footprint. Electrofuels are thus an option for reducinggreenhouse gas emissions from transport, particularly for long-distance freight, marine, and air transport.[60]
The primary targets arebutanol, andbiodiesel, but include other alcohols and carbon-containing gases such asmethane andbutane.
